During pregnancy, early detection of fetal defects is important, especially to older women, many of whom would rather have an abortion than give birth to a baby with a defect such as Downs Syndrome. In any procedure for the detection of early fetal defects it is important that the procedure should involve a minimal risk of miscarriage or of inadvertently inducing abortion. In addition, the test should be performed as soon as possible in the pregnancy, because a decision to abort the pregnancy becomes increasingly difficult, both emotionally and physically with the passage of time. The fetal detection test should also minimize the risk of inadvertently inducing abortion.
Since the 1960's, pregnant women (usually over 35) who have been concerned about the possibility of bearing a child with birth defects, relied on a test called amniocentesis to determine the presence of birth defects. This procedure involves the use of a syringe or large needle to withdraw a small amount of fluid surrounding the fetus from which it can be determined whether defective chromosomes are present. This procedure is typically performed after the fourth month of pregnancy. At this stage the growth of the fetus is so far advanced that a decision to abort can involve significant physical and emotional problems. Investigations of this procedure have produced statistics supporting the suggestion that the procedure causes miscarriages in one of every two hundred women tested. From such investigations it is evidently desirable to be able to detect the presence of fetal defects earlier in the pregnancy and also to reduce the risk of causing a miscarriage or inadvertent abortion. Another problem with amniocentesis is that it takes two to four weeks to grow recovered cells in sufficient quantity for study and consequently even with the earliest possible test a fetus has to be 18 to 20 weeks advanced before this test will yield accurate results.
A procedure which has gained acceptance is Chorionic Villi Sampling (CVS). Chorionic villi are finger-like projections of tissue in the chorionic membrane which eventually forms the placenta. Chorionic villi are well developed around the seventh to eighth weeks of pregnancy. The object of this procedure is to remove, by vacuum, a sample of the villi and assay the sample to determine the genetic health of the fetus. A physician inserts a thin catheter (consisting of a cannula containing an obturator) through the vagina and cervix into the uterus ending at the chorion membrane. When the catheter tip is located on the villi, a source of negative pressure is coupled to the catheter to withdraw a sample of villi tissue for analysis. The advantages of this procedure over amniocentesis is that the biopsies or samples obtained provide enough tissue for analysis, which takes about ten days. Thus, early indications of fetal defects around the seventh to eighth week of pregnancy can be obtained.
To sample chorionic villi it is important that the catheter used is flexible to facilitate insertion yet is curved to conform to the anatomical configuration of the patient. The catheter should have sufficiently rigidity to be handled easily by the physician, and, in addition, once the catheter tip is located on the chorionic villi it is important that when the obturator is withdrawn, the cannula tip remains in the selected location for removing villi from this location in sufficient quantity for analysis. If the tip is moved by the action of withdrawing the obturator, there is a possibility of damage to the chorion membrane if the tip remains in contact, or alternatively the tip may move away from the villi so that the procedure will fail to collect a sample.
One type of existing catheter consists of a hollow flexible cannula and a 1.5 mm diameter flexible aluminum obturator which fits snugly in the cannula to facilitate inserting the cannula. In use, the physician slides the obturator into the cannula and then bends the resulting catheter to obtain the desired degree of bend for insertion. The catheter is then inserted to the desired location and finally the obturator is withdrawn. Although the aluminum obturator is flexible enough to be bent easily by hand, it inevitably retains sufficient rigidity in its bent position to tend to flex the curved portion of the cannula as the curved obturator is removed. As a result the cannula tip is commonly moved from the desired location where it was positioned before the obturator was removed. Apart from this pronounced possibility of tip displacement, it can be difficult to pass the curved obturator through the outer end fitting on the cannula.
Another form of catheter is produced by Downs Surgical plc of England. This product is available through subsidiaries in other countries and consists essentially of a malleable silver cannula which contains a flexible blunt stainless-steel obturator during insertion. The obturator supports the cannula to permit the cannula to be flexed without kinking thereby ensuring that the tubular cannula has internal continuity. The removal of the obturator will clearly affect the cannula because no matter how flexible the stainless-steel obturator is, it will have some stiffness and will affect the shape of the cannula as the obturator is removed so that the end of the cannula will inevitably be removed relative to the position in which it was placed when the obturator was contained in the cannula. Clearly this device also suffers from the aforementioned disadvantages. However it is even less desirable in that the necessary thin wall of the silver cannula has to result in a relatively sharp end which is undesirable in such a device.
Another catheter which uses an internal metallic obturator is designated Model "CV 2" by Lega International Manufacturing & Sales of Chicago, Toronto and Dusseldorf. In this case an aluminum obturator is contained in a flexible cannula of synthetic plastic material and the resulting catheter can be bent into the required shape. The obturator is then removed and of course, because of its strength relative to the plastic cannula, it will cause flexing and will have to be pulled quite firmly out of the cannula because the connections will tend to straighten the obturator.
It is clear from the foregoing that all prior art devices which use an obturator having greater stiffness than the cannula will suffer from the disadvantage of tip displacement when the obturator is removed. In an attempt to overcome this difficulty, a catheter with a preformed curvature was described in U.S. Pat. No. 4,756,708 to the present inventor and this patent issued on July 12, 1988. A precurved cannula received an obturator with a very flexible end which could extend to the end of the cannula to close the catheter during insertion and which could be removed without flexing the curved end portion of the cannula.
It has been found that the structure shown in this U.S. patent is entirely satisfactory in about 80 percent of its uses but that the preset curvature is not the required curvature in about 20 percent of the cases. It is therefore desirable to overcome this disadvantage and that of the other prior art structures by providing a catheter which will be satisfactory in all cases.